Although catalysts for the conversion of hydrocarbons (for example, reforming, dehydrogenation, isomerization, alkylation, and transalkylation) have a tendency to deactivate after a period of use, a catalyst's activity may be restored by one of a number of processes that are known generally as regeneration processes. Regeneration processes are extensively used, and the specific steps included in a regeneration process depend in part on the reason for the deactivation. For example, if the catalyst deactivated because coke deposits accumulated on the catalyst, regeneration usually includes removing the coke by burning. Operating conditions and methods for regeneration processes are well known. Regeneration processes may be carried out in situ, or the catalyst may be withdrawn from the vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for reactivation. Arrangements for continuously or semicontinuously withdrawing catalyst particles from a reaction zone and for reactivation in a regeneration zone are also well known.
Many of these regeneration processes share the common feature of contacting the catalyst in the presence of one or more chlorine-containing species that restore the activity of the catalyst for use in the reaction zone. These chlorine-containing species may be chemically or physically sorbed on the catalyst as chloride or may remain dispersed in a stream that contacts the catalyst. In many regeneration processes, however, a flue gas stream containing the chlorine-containing species is vented from the regeneration process. Several methods have been used for preventing contamination of the flue gas stream with the chlorine-containing species and minimizing the release of the chlorine-containing species in the flue gas stream from the regeneration process. Emissions of chlorine-containing species, apart from the effect of the loss of chloride on the catalyst, pose environmental concerns. The environmental concerns may be abated either by scrubbing the flue gas stream with an aqueous, basic solution that neutralizes the chlorine-containing species or by adsorbing the chlorine-containing species on an adsorbent. In some adsorbent configurations, the conversion catalyst itself (whether in regenerated or deactivated form) may serve as the adsorbent.
The chlorine-removal problem is compounded by water in the flue gas stream, and as a result traditional adsorbents are not economically viable for adsorbing chlorine-containing species from flue gas streams. In order to be economically viable, an adsorbent, while removing a high proportion of the chlorine-containing species from the flue gas stream, must adsorb for example from about 7 to about 8 percent of its weight in chloride. In order to adsorb that much chloride, the flue gas must have a low water content, for example less than about 0.01 mol-% water. Water competes with chlorine-containing species for adsorption sites on the adsorbent, and by occupying sites that would otherwise be occupied by chlorine-containing species, water hinders the adsorption of chloride and hastens replacement of the adsorbent. Thus, if the flue gas has a high water content, the adsorbent adsorbs too much water and is incapable of adsorbing a viable amount of chloride. Because water is a common by-product of coke combustion as a result of the hydrogen-containing compounds that may be found in coke, flue gas streams often do have a high water content, for example from about 1 to about 10 mol-%. As a consequence, unless the flue gas is dried, an adsorbent will adsorb only one-third to one-half of the weight of chloride required for economic viability. This, in turn, may make the adsorption process uneconomical to implement. Although in theory the adsorption of water can be mitigated by drying the flue gas stream prior to adsorbing the chlorine-containing species, in fact a drier is costly as well as impractical because chlorine-containing species such as hydrogen chloride tend to degrade most desiccants.
Accordingly, it is desirable to provide improved methods and systems for preventing the venting of chlorine-containing species during hydrocarbon conversion catalyst regeneration. Additionally, it is desirable to provide such systems and methods that are able to separate the water from the chlorine-containing species to increase the economy of the catalyst regeneration process. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.